Covalent attachment of the hydrophilic polymer, poly(ethylene glycol), commonly referred as PEG, to biologically active agents and surfaces has important applications in biotechnology and medicine.
PEG is generally soluble in water and many organic solvents. PEG is also substantially non-toxic and normally does not elicit any significant immune response in animals. When PEG is chemically attached to a water insoluble compound, the resulting conjugate generally is soluble in water as well as in many organic solvents. When the agent to which PEG is attached is biologically active, such as a drug, the activity of the agent can be retained after the attachment of PEG, and the conjugate generally displays altered pharmacokinetics.
The prodrug approach, in which drugs are released by degradation of more complex agents (prodrugs) under physiological conditions, is a powerful component of drug delivery. See R. B. Greenwald, Exp. Opin. Ther. Patents, 7(6):601-609 (1997). Prodrugs can, for example, be formed by bonding PEG to drugs using linkages that are degradable under physiological conditions.
However, not all linkages are readily degradable and useful in prodrug applications. In general, ester linkages, formed by condensation reactions between PEG carboxylic acids or activated PEG carboxylic acids and alcohol groups on drugs, hydrolyze under physiological conditions to release the drug. For example, in PCT Publication No. WO 96/23794, it is disclosed that paclitaxel can be linked to PEG using ester linkages and the linked paclitaxel can be released in serum by hydrolysis. Antimalarial activity of dihydroartemisinin bonded to PEG through a hydrolyzable ester linkage has also been demonstrated. Bentley et al., Polymer Preprints, 38(1):584 (1997).
Conventional amide and carbamate linkages, formed with amine groups on drugs, generally are stable and do not hydrolyze to release a free drug within a sufficiently short time that is required in practical applications. See, e.g., Zalipsky, Advanced Drug Delivery Reviews, 16:157-182 (1995); Zalipsky, et al., Eur. Polym. J., 19:1177-1183 (1983). For example, it has been demonstrated that carbamate linkages between PEG and a protein in a conjugate are stable under a variety of physiological conditions. Larwood and Szoka, J. Labeled Compd. Radiopharm. 21:603 (1984). Many useful drugs including peptides, proteins, and small agents having amine groups have been bonded to PEG through non-hydrolyzable amide and carbamate linkages. PEG can also be bonded to amine groups on drugs through reductive amination with PEG aldehydes and the resulting amine linkage is non-degradable in vivo.
Because many drugs such as proteins have amine groups that are readily available for reaction to form linkages, it is desirable to make such linkages hydrolytically degradable so that free proteins or other amine-containing agents can be released from the prodrugs at a controlled rate in vivo. Imines, or Schiff bases, offer a possible approach since they hydrolyze to generate the free amine and an aldehyde:RCH═NR′+H2ORCH═O+R′NH2where R′ is a drug or other agent bearing an amino group. This approach has been used in attaching doxorubicin to PEG with release of the drug occurring by hydrolysis of the imine linkage. Ouchi et al. Polymer Preprints, 38(1):582-3 (1997). Since the formation of imines is reversible in water, these compounds are best prepared in organic solvents. Many proteins, peptides, and other agents are thus not amenable to the imine prodrug approach because of their poor solubility or instability in organic solvents.
Conjugates can be prepared by linking an amine-containing drug, through a non-hydrolyzable amide or carbamate linkage, to a PEG molecule having hydrolytically degradable linkages in the PEG backbone. The amine-containing drug is releasable upon the degradation of the PEG backbone. However, the released drug usually has a fragment attached through an amide or carbamate linkage, and the native or parent drug is not released.
U.S. Pat. No. 4,935,465 discloses a water-soluble prodrug in which neighboring group participation by a carboxyl group aids in the hydrolysis of an amide, thus releasing the drug. PEG was a component of a bovine serum albumin (BSA) prodrug disclosed in that patent: 
U.S. Pat. No. 5,561,119 and European Patent No. 595133-A disclose a doxorubicin prodrug as shown below, which utilizes a benzylglucuronyl carbamate linkage. A second component, glucuronidase, must be added in order to cleave the glucuronic acid and liberate doxorubicin and a nitrobenzoquinone methide. 
In yet another approach as disclosed in U.S. Pat. No. 5,413,992, a prodrug of daunamycin shown below, liberates the native drug by an enzyme-induced elimination initiated by abstraction of a proton adjacent to the sulfone group. 
In addition, U.S. Pat. No. 4,760,057 describes enzymatic hydrolysis of a prodrug containing a carbamate linkage:
 RR′NCO2CR1R2O2CR3
where RR′N represents the secondary amine on a drug moiety, and R1-3 are various moieties such as hydrogen, alkyls, or cycloalkyls. Such prodrugs are hydrolyzed by esterases to generate RR′NCO2CR1R2OH, which then decomposes to liberate the drug agent.
Greenwald et al. J. Med. Chem., 42:3657-3667 (1997) discloses prodrugs having a drug linked, through a carbamate linkage to a PEG derivative. 1,4 or 1,6 elimination reaction is required to release the free drug. The prodrug is structurally complex and toxic quinone methide intermediates may be liberated along the free drug.
Thus, the prodrugs in the prior art generally have drawbacks that limit their practical applications. The requirement for enzyme digestion makes the prodrugs unsuitable or at least less useful for in vivo use. In addition, the generation of toxic intermediates can be associated with the release of free drugs. Thus, there remains a need for prodrugs having improved characteristics.